The present invention relates to methods for making semiconductor devices, in particular, semiconductor devices that include high K gate dielectrics.
Metal oxide semiconductor (MOS) field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high dielectric constant dielectric (k) materials, instead of silicon dioxide, can reduce gate leakage. Because such a dielectric may not be compatible with polysilicon, it may be desirable to use metal gate electrodes in devices that include high-k gate dielectrics.
A metal gate electrode may be formed on a high-k dielectric layer by depositing a metal layer on the dielectric layer, masking the metal layer, and then removing the exposed part of that layer. A patterned polysilicon layer may be used to mask the metal layer, and a dry etch process may be used to remove the exposed part of that layer.
During subsequent high temperature steps in the presence of oxygen, the high-k dielectric layer transports oxygen laterally, oxidizing the underlying silicon. This lateral oxidation may result in oxide encroachment under the high-k dielectric. This encroachment increases the effective electrical gate thickness, reducing transistor performance.
Thus, there is a need for better ways to make metal gate/high K transistors.
Features shown in these figures are not intended to be drawn to scale.